let x;
try {
x = ...;
} catch (e) {
return
}
// rest of the code that uses `x`
const y = x + ...;
x只分配了一次,但我必须使用let而不是const.
另一种方式是：
try {
const x = ...;
// rest of the code that uses `x`
const y = x + ...;
} catch (e) {
return
}
有没有更好的办法？
如果尝试失败,我不必关心x的值,因为我将在catch块中返回.
我也不想将其解析为单独的函数.
解决方法
功能 – 使用带有三个回调的辅助函数：
function Try(attempt,onSuccess,onFailure) {
try {
var res = attempt();
} catch(err) {
return onFailure(err);
}
return onSuccess(res);
}
这允许你写
return Try(() => …,x => {
// rest of the code that uses `x`
const y = x + …;
},e => void e);
您还可以使用表示此控制流的数据结构,如Result monad(也称为Either monad)：
class Result {
constructor(go) {
this.go = go;
}
static Ok(v) {
return new this((onSuccess,_) => onSuccess(v));
}
static Err(r) {
return new this((_,onFailure) => onFailure(v));
}
map(f) {
return this.go(v => Result.Ok(f(v)),r => Result.Err(r));
}
chain(f) {
return this.go(v => f(v),r => Result.Err(r));
}
unwrap() {
return this.go(v => v,r => { throw r; });
}
}
function Try(attempt) {
try {
var res = attempt();
return Result.Ok(res);
} catch(e) {
return Result.Err(e);
}
}
你可以使用它与上面简单的辅助函数非常相似：
return Try(() =>
… // exceptions in here are caught
).go(x => {
// rest of the code that uses `x` - exceptions are not caught
const y = x + …;
},e => void e);
但也有更先进的链接：
return Try(() =>
… // exceptions in here are caught
).chain(x =>
Try(() =>
x + … // exceptions in here are caught as well
)
).map(y =>
… // exceptions in here are not caught
).unwrap(); // any caught exceptions are re-thrown
